Why Small Arms Propellants Don't Get Upgraded Often by Ryan Crierie
The big problem with military propellants for small arms is that you are limited to the pressure gradients and envelopes from when that cartridge was developed and adopted for a service weapon.
While Joe Q Hunter can take his .30-06 cartridge and handload it with hotter propellant, and fire it from a custom made beefed up Mauser action; the US Army doesn't have that option, because it would be a nightmare trying to keep track of which weapon can fire what:
DO NOT FIRE 5.56MM M1400 ENHANCED AMMUNITION FROM ANY M-16, M-16A1, M-16A2, M-16A3, OR M-16A4 RIFLE WITH A SERIAL NUMBER OF LESS THAN 34210. WEAPON EXPLOSION DUE TO OVERSTRESS IS PROBABLE.
Propellant advances in military service small arms only happen when a brand new caliber is introduced generally; like for example; the M-14; firing the brand new caliber of 7.62x51 NATO; basically using advances in propellant to shrink the old .30-06 round down significantly, and still retain it's power.
So most likely, if the Army ever choses to formally adopt a 6.8mm or 6.5mm round; it will use advances in propellant technology allowing for a smaller case relative to it's kinetic energy compared to the older 5.56mm rounds (which were designed and specced in the late 50s.)
However; there is one area where propellant technology is constantly updated -- Tank Guns. It's cheaper to modernize a couple thousand tank guns to fire hotter, higher pressure sabot ammunition than it is to modernize 1,000,000 M-16s to the same level.